Recently, X-ray CT apparatuses that perform photon counting CT using a photon-counting mode detector are being developed. Unlike an integral mode detector used in a conventional X-ray CT apparatus, the photon-counting mode detector outputs a signal from which X-ray photons that have passed through a subject can be individually counted. Therefore, in the photon counting CT, an X-ray CT image having a high signal-to-noise (S/N) ratio can be reconstructed.
Moreover, the signal output by the photon-counting mode detector can be used for measurement (discrimination) of the energy of an X-ray photon. Therefore, in photon counting CT, data that is collected by irradiating X-rays with one tube voltage can be separated into energy components to be made into images. For example, in photon counting CT, an image that enables identification of a material using a difference in the K absorption edge can be generated.
However, in photon counting CT, noise components such as scattered radiation is also counted, and therefore, the quality of reconstructed images can be deteriorated.